Yersinia pestis is the causative agent of bubonic and pneumonic plague. Bacillus anthracis is the causative agent for the anthrax disease. The Centers for Disease Control and Prevention (hereinafter “CDC”) lists Y. pestis and B. anthracis as two of the six Category A biological agents that pose a risk to national security.
Yersinia pestis 
The etiologic agent of plague is the Gram-negative bacterium Yersinia pestis. The natural route of transmission of Y. pestis from one animal host to another is either directly or via a flea vector. Plague is endemic in some regions of the world and outbreaks occasionally occur as a consequence of natural disasters. Y. pestis is also a concern as one of the microorganisms with potential for use against civilian or military populations as a biological warfare/biological terrorism agent. In such a situation, the pneumonic form of plague would be the most likely outcome. This form of plague is particularly devastating because of the rapidity of onset, the high mortality, and the rapid spread of the disease. Immunization against aerosolized plague presents a particular challenge for vaccine developers. There is currently no vaccine for plague.
Both live attenuated and killed plague vaccines have been used in man, although questions remain about their safety and relative efficacy, especially against the pneumonic form of infection. Since plague remains endemic in some regions of the world, and because of the possibility of the illegitimate use of Y. pestis as a biological warfare agent, development of improved vaccines against plague is a high priority. The ideal vaccine should be deliverable in a minimum of doses and quickly produce high titer and long-lasting antibodies. Moreover, such a vaccine should protect against aerosolized transmission of Y. pestis. 
The two most recently described approaches to development of improved plague vaccines are 1) attenuated mutants of Y. pestis and 2) subunit vaccines. The potential efficacy of attenuated mutants of Y. pestis as vaccines is supported by experience with the live attenuated vaccine strain EV76. This vaccine has been in use since 1908 and is given as a single dose Immunization of mice with EV76 induces an immune response and protects mice against subcutaneous and inhalation (aerosolized) infection. However, this vaccine strain is not avirulent and has an unacceptable safety profile. Moreover, multiple variants of the classical EV76 strain exist that differ significantly in passage history and genetic characteristics. Recent studies have focused on creating defined genetically attenuated mutants of Y. pestis, similar to those created in other Gram-negative bacteria (i.e., Salmonella spp.). For unknown reasons, genetic mutations, which markedly attenuate Salmonella spp. do not attenuate Y. pestis. For instance, an aroA mutant of Y. pestis was fully virulent in the murine model of disease but avirulent in guinea pigs.
A number of potential subunit vaccines have been evaluated for immunogenicity and protective efficacy against Y. pestis. The two most promising are F1 and V. F1 is a capsular protein located on the surface of the bacterium and the V antigen is a component of the Y. pestis Type III secretion system. These proteins have been produced recombinantly and induce protective immune responses when administered individually. A combination or fusion of F1 and V may have an additive protective effect when used to immunize humans against plague. It is thought that F1-V fusion protein should provide protection against both subcutaneous and aerosol challenge, and will have the potential to provide protective immunity against pneumonic as well as bubonic plague due to either wild type F1+ Y. pestis or to naturally occurring F1-variants. To date no one has been able to express the F1-V fusion protein in transgenic chloroplast. Such an accomplishment would provide a large supply of high-quality antigen for vaccines.
Bacillus anthracis 
Bacillus anthracis is the organism that causes the anthrax disease. It is a Gram-positive, nonmotile, aerobic or facultatively anaerobic, spore-forming bacterium. The spores are about 1 μm in size, extremely hardy, resistant to gamma rays, UV light, drying, heat, and many disinfectants. Spores germinate upon entering an environment rich in glucose, amino acids, and nucleosides, such as in animal and human tissues and blood. The vegetative cells enter the spore state when the nutrients are exhausted or when the organisms are exposed to molecular oxygen in the air.
Anthrax is typically a disease of animals, especially herbivores such as cows, sheep, and goats. It affects humans through contact with the spores in one of three ways. Cutaneous anthrax occurs when the spores enter the body through a cut or an abrasion on the skin. Gastrointestinal anthrax occurs when the spores enters the body through consumption of contaminated meat products. Inhalation anthrax occurs when the spores enter the body through inhalation of the spores.
When spores enter the body, macrophages engulf them, migrate to regional lymph nodes and the spores germinate into vegetative bacteria. Macrophages release the vegetative bacteria and they spread through the blood and lymph until there are up to 108 bacilli per milliliter of blood. The exotoxins are produced from bacteria and they lead to symptoms and possible death. Spores can survive in the lungs or lymph nodes up to 60 days before germination occurs. In animal experiments, it has been seen that once toxin secretion has reached a critical threshold, death will occur, even if the blood is rendered sterile through the use of antibiotics. From primate studies, the estimated lethal dose of inhaled anthrax spores sufficient to kill 50% of humans exposed to it (the LD50) is 2,500-55,000 spores.
The CDC lists anthrax as a category A disease agent and estimates the cost of an anthrax attack would be $26.2 billion per 100,000 persons exposed. The only vaccine licensed for human use in the U.S., Biothrax (formerly Anthrax vaccine adsorbed, or AVA), is an aluminum hydroxide-adsorbed, formalin-treated culture supernatant of a toxigenic, nonencapsulated, non-proteolytic strain of Bacillus anthracis. In addition to the immunogenic protective antigen (PA), the vaccine contains trace amounts of edema factor (EF) and lethal factor (LF) that may contribute to the local reactions seen in 5-7% of vaccine recipients, or reported to be toxic causing side-effects. There is a clear need and urgency for an improved vaccine for anthrax and for improved production methods that allow it to be mass-produced at reasonable cost.
There are two main virulence factors associated with B. anthracis, the polyglutamyl capsule which is believed to prevent the vegetative bacterial cells from being phagocytized and the exotoxins. Two different exotoxins are produced by three factors. PA binds to the host cell, LF is a zinc metalloprotease which inactivates mitogen-activated protein kinase. The edema toxin is formed when PA binds to EF. This toxin increases cyclic AMP (cAMP) levels in the cell which upsets the water homeostasis resulting in accumulation of fluid called edema. The lethal toxin is formed from binding of PA and LF. This toxin stimulates macrophages to release interleukin-1b, tumor necrosis factor a, and other cytokines which contribute to shock and sudden death.
Anthrax has become a serious threat due to its potential use in bioterrorism and recent outbreaks among wild-life in the United States. Concerns regarding vaccine purity, the current requirement for six injections followed by yearly boosters, and a limited supply of the key protective antigen (PA), underscore the urgent need for an improved vaccine.